the aid of the microscope.

Wolff furnished the conclusive empirical proof of his theory in his
classic dissertation on The Formation of the Alimentary Canal (1768).
In its complete state the alimentary canal of the hen is a long and
complex tube, with which the lungs, liver, salivary glands, and many
other small glands, are connected. Wolff showed that in the early
stages of the embryonic chick there is no trace whatever of this
complicated tube with all its dependencies, but instead of it only a
flat, leaf-shaped body; that, in fact, the whole embryo has at first
the appearance of a flat, oval-shaped leaf. When we remember how
difficult the exact observation of so fine and delicate a structure as
the early leaf-shaped body of the chick must have been with the poor
microscopes then in use, we must admire the rare faculty for
observation which enabled Wolff to make the most important discoveries
in this most difficult part of embryology. By this laborious research
he reached the correct opinion that the embryonic body of all the
higher animals, such as the birds, is for some time merely a flat,
thin, leaf-shaped disk--consisting at first of one layer, but
afterwards of several. The lowest of these layers is the alimentary
canal, and Wolff followed its development from its commencement to its
completion. He showed how this leaf-shaped structure first turns into
a groove, then the margins of this groove fold together and form a
closed canal, and at length the two external openings of the tube (the
mouth and anus) appear.

Moreover, the important fact that the other systems of organs are
developed in the same way, from tubes formed out of simple layers, did
not escape Wolff. The nerveless system, muscular system, and vascular
(blood-vessel) system, with all the organs appertaining thereto, are,